AC motors are currently powered from a DC supply bus at the edges of which are connected the different phases of the motors via a frequency converter, consisting of switching units, the state of which is driven by suitable control electronics.
Such control electronics are for example discussed in documents EP-A-0 458 511 and DE-A-141 35 26.
For motors driving pumps or very powerful compressors, notably for the oil industry, very great electric powers may circulate through switching units ensuring the powering of the motor. As an example, voltages comprised between 1,000 and 4,000 volts may be applied on the switching units.
The switching units are formed with transistors of the insulated gate bipolar type. The insulated gate bipolar transistor, with the acronym of IGBT, is a semi-conductor device from the family of transistors, which is used as an electronic switch, mainly in power electronic circuits. This component, which combines the advantages of great simplicity for controlling the field effect transistor as compared with the bipolar transistor while retaining the low losses by combustion of the latter, has allowed much progress in power electronics both as regards liability and the economic aspect. Nevertheless, in order to make frequency converters for very powerful motors, a plurality of transistors of the IGBT transistor type should be mounted in series for ensuring each switching, a single transistor not giving the possibility of interrupting circulation of the current for very high voltages on its terminals.
The plurality of transistors, thereby connected in series, should be controlled with perfect synchronization so that the transistors functionally correspond to a single transistor. Synchronization errors between the different transistors lead to the fact that the voltage is supported by only one of the transistors, which may cause its destruction.
Taking into account the manufacturing tolerances of the transistors and the accuracy of the control electronics, synchronization of the operation of the transistors is delicate to ensure. Highly sophisticated control electronic solutions have thus been developed. Such solutions involve the application of specific integrated circuits which are very expensive.
A technique for protecting the transistors, currently designated by the expression «active clamping» is also known. According to this solution, the transistors are associated between the control gate and a conducting terminal with a Zener diode with a resistance which, when the voltage is too strong between the conducting terminals of the transistor, gives the possibility of bringing the transistor back to its conducting state and thereby limiting the voltage on its terminals.
This technique imposes very high margins for igniting the transistors and imposes the use of resistant and relatively accurate Zener diodes.
Document WO-A-2007/122 322 also describes a method for balancing the voltages of two IGBT transistors placed in series using a so-called «active sharing» method (master/slave notion).
These different solutions prove to be suitable for a limited number of IGBT transistors, such as two or three. However, when a large number of IGBT transistors is considered for putting them in series, notably four of them, these solutions are no longer suitable, notably because the proposed adjustment is dependent both on the transistor and on the relevant control circuit. In the case of a replacement following malfunction of one of the two elements, it is the whole of the adjustment which has to be taken up again.
Therefore, there exists a need for a circuit giving the possibility of facilitating maintenance of the systems in which power transistors, in particular IGBT transistors, are triggered simultaneously, notably in a series configuration.